A method of preparing polymer particles having internal vesicles, as a replacement of titanium dioxide used as a white pigment in manufacturing water-born paints or paper coatings for cost reduction, or improvement in gloss and processibility, is publicly known.
U.S. Pat. No. 3,615,972 discloses a method of preparing vesiculated particles by first manufacturing the plastic particles containing the foaming agents and then foaming by heat. U.S. Pat. No. 3,891,577 discloses a preparation method of vesiculated polyester particles of 1.about.25 microns of diameters by the so-called double emulsion method that utilizes the form "water/oil/water emulsion." Japanese Pat. Pub. No. 85-252635 discloses a method of producing solvent-containing particles, after which are subjected to vaporization swelling. However, all of these inventions suffer from such drawbacks as difficulties in preparation of the uniform particles, large-sized particle diameters, a broad distribution of variation in particle diameters, poor synthetic stability, poor storage stability, and economical inefficiency for commercial mass production.
U.S. Pat. No. 4,427,836, which has solved such problems, provides a method of preparing vesiculated particles having closed vesicles, wherein particles are first produced by the core-shell multistage emulsion polymerization process in which particles of alkali-swellable cores with high contents of acid monomers are encapsulated by oleophilic polymers with little acid monomers, and then the water inside the particles is released during the drying process. In the case of such polymerization process of the form of a "hydrophilic core-hydrophobic shell," a phase inversion is much likely to occur on the interface between the core and the shell. In other words, since a hydrophobic core-hydrophilic shell is more stable thermodynamically, the phase inversion would naturally take place. In order to prevent this phase inversion, the composition of the monomer mixture, the glass transition temperatures of the polymers formed, the types and quantities of emulsifiers, the types and quantities of polymerization initiators, reaction temperatures etc. must be governed in an optimum way at each stage of the polymerization process. To this end, the method suffers from the defects caused by these restrictions such as the narrow limitation of product compositions, the complicated and inefficient processes, and the like. Particularly, if the hydrophilicity of the core-forming polymer positioned in the center of the particle is far greater than that of the shell-forming polymer constituting the sheath of the particles, there arises an unavoidable problem that at least one step for forming the shell must be added to enhance the efficiency of encapsulation. Another problem exists in that although the core particle should contain a high amount of acid monomers in order to maximize the efficiency of alkali-swelling, the polymerization stability becomes poor, and the efficiency of encapsulation in the subsequent process declines if the amount of acid monomers in the core particles is over 40% by weight. To this end, the amount of acid monomers had to be limited, thereby causing a defect of failure to maximize the efficiency of alkali swelling. Moreover, there was also a problem of an unnecessarily thickened shell. To be specific, the wall of the finally produced vesiculated particle becomes inevitably thicker than it may be needed because if it is desirable to encapsulate the hydrophilic core, the shell-forming monomers should be used at least 4 times, preferably 8 times, as much as the core-forming monomers in order to obtain the concentric core-shell particle structure.
U.S. Pat. No. 5,639,805, which was an improvement of the above, used a feed method of gradually changing the composition of the monomer mixture in the shell-forming stage, thereby minimizing the wall thickness of hollow polymer particles, and as a result, contributing to cost-reduction by decreasing the amount of polymeric materials used. In this case, too, however, the decrease in productivity could not be avoided in this multistage emulsion polymerization process.
U.S. Pat. No. 5,494,971 discloses a process without the use of a tie-coat layer which is separately introduced to buffer the differences in hydrophilicity between the core particle polymers and the monomer compositions used in the shell emulsion polymerization process. In this process, the acid monomer is added separately and concurrently in the initial stage of formation of the main shell polymerization in order to eliminate the need for a tie-coat layer, thereby naturally forming a polymer of intermediate hydrophilicity on the interface between the core and the shell. In this way, this process could reduce one step among many steps in the polymerization process. However, this process had a weak point in that the time required for completion of the polymerization could not be sufficiently reduced due to the adoption of the two-stage polymerization process.
U.S. Pat. No. 4,973,670 discloses a single-stage method of preparation characterized in that a single-stage emulsion polymerization process is carried out in the presence of an organic solvent, and then the emulsion polymer so produced is dried to develop the particles having vesiculated structures. This method, however, suffers from the fact that it could not develop the particles of uniform interior structure because it was not easy to control the organic solvent of high fluidity to be positioned at the center of the particle during the phase separation process of the polymer--the separation process being performed concurrently in the course of polymerization process.
As described above, most of the conventional processes of preparing emulsion polymer particles suitable for use as an opacifying agent are the ones using the core-shell polymerization method. However, in such processes, it is difficult to control the interior structure of the particles as intended due to the considerable differences in hydrophilicity between each polymer layer, thereby the concentration of the acid monomer of the core polymer having to be restricted to less than 40% by weight; when the water is removed during the drying process of the hydrated swollen polymer particle, it is much probable that the wall of the particle collapses by the volume contraction force applied to the interface between the swollen core polymer and the hydrophobic shell polymer; the eccentric core-shell particles, which are partially formed in the course of encapsulating the exterior of the hydrophilic core particle with the hydrophobic polymer, form the vesicles of opened forms, which is problematic, when the hydrophobic polymer inside the particle is released out of the particle during the neutralization/swelling process.